Re: [PATCHv3 19/27] timens/fs/proc: Introduce /proc/pid/timens_offsets

[Andrei Vagin <avagin@gmail.com>]

Hi Jann,

Thank you for the review. Here are a few comments inline.

On Thu, Apr 25, 2019 at 08:16:41PM +0200, Jann Horn wrote:
> On Thu, Apr 25, 2019 at 6:15 PM Dmitry Safonov <dima@arista.com> wrote:
> > API to set time namespace offsets for children processes, i.e.:
> > echo "clockid off_ses off_nsec" > /proc/self/timens_offsets
> [...]
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 6a803a0b75df..76d58e9b5178 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> [...]
> > @@ -1521,6 +1522,103 @@ static const struct file_operations proc_pid_sched_autogroup_operations = {
> >
> >  #endif /* CONFIG_SCHED_AUTOGROUP */
> >
> > +#ifdef CONFIG_TIME_NS
> > +static int timens_offsets_show(struct seq_file *m, void *v)
> > +{
> > +       struct inode *inode = m->private;
> > +       struct task_struct *p;
> > +
> > +       p = get_proc_task(inode);
> 
> (FYI, this could also be "p = get_proc_task(file_inode(m->file));".
> But this works, too.)
> 
> > +       if (!p)
> > +               return -ESRCH;
> > +       proc_timens_show_offsets(p, m);
> > +
> > +       put_task_struct(p);
> > +
> > +       return 0;
> > +}
> > +
> > +static ssize_t
> > +timens_offsets_write(struct file *file, const char __user *buf,
> > +           size_t count, loff_t *ppos)
> > +{
> > +       struct inode *inode = file_inode(file);
> > +       struct proc_timens_offset offsets[2];
> > +       char *kbuf = NULL, *pos, *next_line;
> > +       struct task_struct *p;
> > +       int ret, noffsets;
> > +
> > +       /* Only allow < page size writes at the beginning of the file */
> > +       if ((*ppos != 0) || (count >= PAGE_SIZE))
> > +               return -EINVAL;
> > +
> > +       /* Slurp in the user data */
> > +       kbuf = memdup_user_nul(buf, count);
> > +       if (IS_ERR(kbuf))
> > +               return PTR_ERR(kbuf);
> > +
> > +       /* Parse the user data */
> > +       ret = -EINVAL;
> > +       noffsets = 0;
> > +       pos = kbuf;
> > +       for (; pos; pos = next_line) {
> > +               struct proc_timens_offset *off = &offsets[noffsets];
> > +               int err;
> > +
> > +               /* Find the end of line and ensure I don't look past it */
> > +               next_line = strchr(pos, '\n');
> > +               if (next_line) {
> > +                       *next_line = '\0';
> > +                       next_line++;
> > +                       if (*next_line == '\0')
> > +                               next_line = NULL;
> > +               }
> > +
> > +               err = sscanf(pos, "%u %lld %lu", &off->clockid,
> > +                               &off->val.tv_sec, &off->val.tv_nsec);
> > +               if (err != 3 || off->val.tv_nsec >= NSEC_PER_SEC)
> > +                       goto out;
> > +               if (noffsets++ == ARRAY_SIZE(offsets))
> > +                       break;
> 
> This is equivalent to:
> 
> if (noffsets == ARRAY_SIZE(offsets)) {
	  noffsets++;
>         break;
  }
> noffsets++;
> 
> So we can reach the start of the loop with
> noffsets==ARRAY_SIZE(offsets), right? Which means that an
> out-of-bounds write can happen?

good catch. I will add a test for this case.

> 
> I think that for code like this, it makes sense to write the increment
> and the check out separately so that it's easier to spot problems;
> e.g. like this:
> 
> noffsets++;
> if (noffsets == ARRAY_SIZE(offsets))
>         break;

will rewrite this way. Thanks!

> 
> > +       }
> > +
> > +       ret = -ESRCH;
> > +       p = get_proc_task(inode);
> > +       if (!p)
> > +               goto out;
> > +       ret = proc_timens_set_offset(p, offsets, noffsets);
> > +       put_task_struct(p);
> > +       if (ret)
> > +               goto out;
> > +
> > +       ret = count;
> > +out:
> > +       kfree(kbuf);
> > +       return ret;
> > +}
> > +
> > +static int timens_offsets_open(struct inode *inode, struct file *filp)
> > +{
> > +       int ret;
> > +
> > +       ret = single_open(filp, timens_offsets_show, NULL);
> > +       if (!ret) {
> > +               struct seq_file *m = filp->private_data;
> > +
> > +               m->private = inode;
> > +       }
> > +       return ret;
> > +}
> 
> Why did you write it like this? Wouldn't the following be equivalent?

Probably I looked at sched_autogroup_open

> 
> static int timens_offsets_open(struct inode *inode, struct file *file)
> {
>         return single_open(file, timens_offsets_show, inode);
> }
> 
> (But also, you can reach the inode of a seq_file as file_inode(m->file).)
> 
> [...]
> > diff --git a/kernel/time_namespace.c b/kernel/time_namespace.c
> > index e806accc4eaf..9ad4b63c4ed2 100644
> > --- a/kernel/time_namespace.c
> > +++ b/kernel/time_namespace.c
> [...]
> > +
> > +int proc_timens_set_offset(struct task_struct *p,
> > +                          struct proc_timens_offset *offsets, int noffsets)
> > +{
> > +       struct ns_common *ns;
> > +       struct time_namespace *time_ns;
> > +       struct timens_offsets *ns_offsets;
> > +       struct timespec64 *offset;
> > +       struct timespec64 tp;
> > +       int i, err;
> > +
> > +       ns = timens_for_children_get(p);
> > +       if (!ns)
> > +               return -ESRCH;
> > +       time_ns = to_time_ns(ns);
> > +
> > +       if (!time_ns->offsets || time_ns->initialized ||
> > +           !ns_capable(time_ns->user_ns, CAP_SYS_TIME)) {
> 
> Capability checks in VFS read/write handlers are bad. Please pass
> through the file pointer to this function and replace the call with
> "file_ns_capable(file, time_ns->user_ns, CAP_SYS_TIME)".

Will fix. Thanks!
> 
> > +               put_time_ns(time_ns);
> > +               return -EPERM;
> > +       }
> [...]
> > +}